Remove readme comments on old version support
Add note about supported versions to README and CONTRIBUTING.md
Remove 1.8.7 warning
Add required_ruby_version to gemspec
Remove check for Rails > 3
Drop 1.8 json gemspec check
Require engine if Rails is defined
Add readme note about support for Ruby 1.8.x and Rails 2.3
Merge pull request #271 from qpowell/remove-old-version-support

Remove support for Ruby 1.8.7 and Rails 2.3